The present invention relates to ah optical disc recording and reproducing apparatus that reproduces information recorded in an optical disc, and more particularly, to a process for correcting light reception deviation of a photo detector due to lens shift in a search operation of an optical pickup.
Generally, in optical disc reproducing apparatuses, a tracking servo operation for an optical pickup is performed based on a tracking error signal. More particularly, the tracking servo operation is performed in accordance with a track cross signal of which a pulse width changes in accordance with the amount (the amount of deviation on the inner peripheral side or outer peripheral side of a track on the optical disc) of shift of an objective lens included in an optical pickup with respect to a thread in which the optical pickup is built.
For example, when the objective lens is shifted to the inner peripheral side of a track on the optical disc, the location of the objective lens is controlled to be in a normal position with respect to the track by applying a tracking drive voltage to a tracking actuator such that the objective lens is moved in the direction of the outer peripheral side for returning the objective lens to a normal position with respect to the track by a tracking servo on the basis of the track cross signal at that moment.
However, in the general optical disc reproducing apparatuses, the following process is performed by a system controller until the focus servo is turned on. Here, the process will be explained assuming that a DVD is inserted into the optical disc reproducing apparatus.
The system controller acquires a sum signal of (A+B+C+D) and a focus error signal (A+C)-(B+D) from a four-division photo detector in a state that a DVD laser diode is turned on. Then, the system controller acquires a sum signal of (A+B+C+D) and a focus error signal (A+C)-(B+D) in a state that the DVD laser diode is turned off and a CD laser diode is turned on, similarly.
Then, the system controller determines that the inserted disc is a DVD, if the level of disc type determination signal ((A+C)-(B+D))/(A+B+C+D) acquired in a state that the DVD laser diode is turned on is higher than the level of the disc type determination signal ((A+C)-(B+D))/(A+B+C+D) acquired in a state that the CD laser diode is turned on. Then the system controller individual the levels of detection signals A, B, C, and D to be a constant level, which have been transmitted from the four-division photo detector in a state that the DVD laser diode is turned on. Next, the system controller performs an S-letter balance adjustment operation for the focus error signal of (A+C)-(B+D) acquired in a state that the DVD laser diode is turned on and a focus lead-in setting operation for leading in the focus servo. Thereafter the system controller turns on the focus servo.
In the general optical disc reproducing apparatuses, the spot of a laser beam emitted from the optical pickup is controlled to be in a track-on state for a track on an optical disc by turning on the focus servo for performing a focus-on operation by the processes described above and then turning on the tracking servo. However, when a de-track (a state that a track-on operation is performed from a location deviated from the center of a track) occurs in a process up to the above-described process for turning on the focus servo, even if the focus-on operation is performed in the state, a laser beam is projected onto a location deviated from the center of the track. As a result, the jitter of a reproduction signal is deteriorated, and accordingly, there is a problem that the reproduction quality of the optical disc is degraded. In addition, for a next-generation DVD such as a Blu-ray DISC or an HD DVD that has become widespread in the market, the memory capacity is used at high precision, and accordingly, the track pitch thereof is narrowed markedly, compared to that of a general optical disc such as a CD or a DVD. As a result, high-precision tracking is desired.
In addition, in general technology disclosed in Patent Document 1, in various optical reproducing apparatuses, means in which a tangential phase difference is intentionally deviated, and the amount of lens shift is detected based on an offset of the phase difference and is corrected, and a balance adjustment operation of the tracking error is performed is disclosed. According to the technology disclosed in Patent Document 1, a minute deviation of tracking that frequently occurs in a reproduction process can be corrected. However, it is difficult to correct deviation of an optical spot generated in a manufacturing process of each optical pickup.
Patent Document 1: Japanese Patent Publication No. 2000-315327A